1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a semiconductor device having a function of rapidly turning off upon detection of an abnormality.
2. Description of Related Art
In recent years, semiconductor devices have been used in systems having a switch function capable of driving a large current, as typified by electrical components for automobiles. Each system that drives a large current has a function of switching itself off upon detection of an overheat, an overcurrent, or the like so as to protect the system when an abnormality occurs in which a load is short-circuited and a large current flows, for example.
In the operation in which the system switches itself off upon detection of an abnormality, it is important to reduce a heat loss that occurs upon turn-off. This is because if a large heat loss occurs upon turn-off, a breakdown may occur, since it is highly possible that a large amount of heat is generated when an abnormality is detected. Accordingly, in the turn-off operation upon detection of an abnormality, it is important to rapidly turn off.
Japanese Unexamined Patent Application Publication No. 2005-123666 (Kojima et al.) discloses a technique for an output circuit that rapidly turns off. FIG. 7 is a diagram showing the configuration of the output circuit according to the technique disclosed by Kojima et al. Referring to FIG. 7, an output circuit 300 includes an output MOS 100 which is connected between a power supply terminal and an output terminal. The output MOS 100 is an N-type enhancement transistor having a circuit switching function. A transistor Q1 and a transistor Q2 are N-type depletion or enhancement transistors. A transistor Q3 is an N-type enhancement transistor. A resistor R1 is connected between the transistor Q1 and the output terminal, and a resistor R2 is connected between the transistor Q2 and the output terminal. Thus, the transistors Q1 to Q3 form three discharge paths between the gate terminal of the output MOS 100 and the output terminal. The output circuit 300 also includes a state determination circuit 304 that controls the discharge of an electric charge stored in the gate electrode of the output MOS 100 by selecting the discharge paths. The output circuit 300 further includes a control signal input circuit 301, a booster circuit 302, and a rise rate 303, which are blocks for controlling the output MOS 100 in the normal state.
In the normal state, the control signal input circuit 301 receives a signal for turning on and off the output MOS 100. Based on the signal, the booster circuit 302 outputs a boosted voltage to the gate terminal through the rise rate 303. In the case of turning off the output MOS in the normal state, the booster circuit 302 suspends operation and the state determination circuit 304 detects a voltage between the power supply terminal and the output terminal, based on the signal output from the control signal input circuit 301. The output circuit 300 includes a discharge path formed of the transistor Q1 and the resistor R1, a discharge path formed of the transistor Q2 and the resistor R2, and a discharge path formed of the transistor Q3. Based on the signal output from the state determination circuit 304, one or more discharge paths are activated to discharge a gate charge of the output MOS to the output terminal, thereby turning off the output MOS.
The state determination circuit 304 measures a voltage between the power supply terminal and the output terminal and detects whether an overcurrent flows, for example, thereby detecting an abnormality. In this case, an OFF signal for forcibly turning off the MOS 100 is input to the control signal input circuit 301. Further, based on the signal output from the state determination circuit 304, one or more of the three discharge paths are activated to discharge the gate charge of the output MOS to the output terminal, thereby turning off the output MOS.
In the turn-off operation in the normal state, the transistor Q1 is activated. In a rapid turn-off operation in an overcurrent state, only the transistor Q3 or both the transistor Q1 and the transistor Q2 are activated at the same time. In the turn-off operation when a current which is larger than that in the normal state and smaller than an overcurrent flows, only the transistor Q2 or both the transistor Q2 and the transistor Q1 are activated at the same time, thereby adjusting the slew rate.
In the overcurrent state, the operation of the transistor Q3, which is an enhancement transistor, is especially important. The rapid turn-off operation is accomplished in such a manner that the transistor Q3 rapidly discharges the gate charge of the output MOS. Next, the operation of the transistor Q3 will be described in detail. Herein, a description is given to the following two cases:    (1) a case where the rapid turn-off operation is accomplished by activating only the transistor Q3; and    (2) a case where the rapid turn-off operation is accomplished by activating all the transistors Q1 to Q3.
(1) The Case where the Rapid Turn-Off Operation is Accomplished by Activating only the Transistor Q3
Assuming that a gate-source voltage of the transistor Q3 is Vgs3 and a threshold voltage is Vthn, Vgs3>Vthn should be satisfied in order to turn on the transistor Q3. Assuming that a gate voltage is Vx, the threshold voltage of the transistor Q3 is Vthn, and an output voltage is OUT, Vx−OUT>Vthn is established, so it is necessary to apply a signal having a potential level, which satisfies Vx>OUT+Vthn, to the gate. Also, it is necessary to generate the signal in the state determination circuit.
Assuming herein that a voltage at the power supply terminal is VCC, an ON-resistance of the output MOS is Ron, and a current driven by the output MOS is lout, an output potential OUT is given by OUT=VCC−Iout·Ron. For example, assuming that VCC=12 V, Iout=70 A, Ron=10 mΩ, and Vthn=1 V, OUT=12 V−70 A×10 mω=11.3 V is established, and therefore Vx>11.3 V+1 V=12.3 V. Accordingly, it is necessary that the gate voltage Vx be equal to or higher than the power supply voltage VCC. This is merely an example, and even if the voltage equal to or higher than the power supply voltage VCC is not applied, the transistor Q3 can be turned on. In the case where the transistor Q3 turns on at a voltage equal to or lower than the power supply voltage VCC, the transistor Q3 operates in a saturation region at the start of the turn-off operation, with the result that the discharge current is limited immediately after the discharge operation is started. In order to achieve the rapid discharge operation using only the transistor Q3 immediately after the start of the discharge operation, it is necessary to apply a high voltage to the gate so that the transistor Q3 can sufficiently drive the current. Also, it is necessary to apply a voltage equal to or higher than the power supply voltage VCC to the gate voltage Vx. Therefore, a booster circuit such as a bootstrap is required.
(2) The Case where the Rapid Turn-Off Operation is Accomplished by Activating all the Transistors Q1 to Q3
A potential equal to the power supply voltage level is applied to the gate of the transistor Q3, and the transistors Q1 and Q2 are activated at the same time. In the case where the transistors Q1 and Q2 are depletion transistors, the transistors Q1 and Q2 turn on upon application of a potential equal to the power supply voltage VCC or the output voltage OUT. When the transistors Q1 and Q2 discharge the gate charge of the output MOS and a predetermined amount of electric charge is discharged, the output potential OUT starts to decrease. This enables the transistor Q3 to turn on. Further, when the output voltage OUT decreases with the progress of the discharge operation and when Vgs3>Vds3−Vthn, i.e., Vx−OUT>GATE−OUT+Vthn (where GATE represents the potential of the gate electrode of the output MOS) is satisfied, the transistor Q3 can cause a large discharge current to flow, because the transistor Q3 operates in a linear region.
Thus, when the rapid discharge operation is accomplished by using the transistors Q1 to Q3, the transistor Q3 is not necessarily turned on after the turn-off operation. Therefore, it is sufficient that the voltage GATE to be applied to the gate is equal to the power supply voltage VCC. This eliminates the need to provide a special circuit such as a bootstrap. However, in order to increase the discharge current, it is necessary to increase the size of the transistors, since the transistors Q1 and Q2 carry out the discharge operation immediately after the turn-off operation and the transistors Q1 and Q2 are depletion transistors.